More solar radiation reaches the lower latitudes (close to the equator) than the higher latitudes (close to the poles).

"Once that north south difference becomes large enough, the atmosphere becomes unstable," says Thompson.

This drives the creation of storms in the mid-latitudes, which move warm air towards the poles and reduces the differential in heat that builds up between the sub-tropics and the polar regions.

"The storms will reduce that north-south difference in temperature and the flow becomes more stable," says Thompson. "But then over time it will become more and more unstable and there will a period of storms again."

Thompson and Barnes found that in the Southern Hemisphere this cycle repeats every 20 to 30 days.

Climate observations

The researchers' findings come from an analysis of climate observations between 1979 and 2012.

They looked at measurements of the kinetic energy associated with the motion of the storms, the heat they move towards the poles, and the precipitation they bring.

"In the Southern Hemisphere the energy associated with the storms and their movement of heat towards the poles has this rhythmic quality to it every 20 to 30 days," says Thompson.

"We have not been able to establish a similar rhythmic variability in the Northern Hemisphere."

Thompson says this could be because the Southern Ocean has relatively less landmass to disrupt the storms.

"Storms in the North Atlantic move over Europe and into Russia and get disrupted by the mountains and the continents," he says.

While the findings could be relevant to weather forecasting, Thompson emphasises that the findings are based on an average over the entire Southern Hemisphere mid-latitudes.

"To what extent that rhythm shows up at the local scale is something we haven't studied yet," he says.

Still, he says, apart from the seasonal and daily cycles, there are few other weather cycles that are driven by the climate system itself and these tend to be in the tropics, says Thompson.

"There are not many phenomena that are periodic in the climate system and to me and that's why this is fun to work on," he says.

Australian climate scientist, Professor Matthew England of the Climate Change Research Centre at University of New South Wales welcomes the research.

"We can predict weather pretty well out to a week and two weeks is generally always considered the very upper limit because processes kick in that are chaotic and intrinsically difficult to predict," he says.

"So any piece of research that deepens our understanding of the 20- to 30-day cycles that can occur in the atmosphere like this paper has done has really important implications for weather forecasting."